1. Field of the Invention
This invention relates to a hydraulic tilt device for a marine outboard drive and more particularly to an improved hydraulic tilt device that affords good operational performance for a relatively small outboard drive.
2. Description of Related Art
Marine outboard drives, i.e., both outboard motors and the outboard drive sections (stern drives) of inboard motors, have a wide variety of hydraulic tilt devices that support the outboard drives for tilting movement relative to an associated watercraft about a generally horizontally disposed tilt axis. The hydraulic tilt device generally comprises a cylinder, a piston slidably supported in the cylinder and defining a pair of fluid chambers and a piston rod extending from the piston through one of the fluid chambers. The fluid chambers are filled with a working fluid. Either one of the cylinder or piston rod is affixed to a cramp bracket and the other one is affixed to a swivel bracket. The cramp bracket is mounted on the associated watercraft, while the swivel bracket directly supporting the outboard drive is pivotally mounted on the cramp bracket about a tilt axis. Accordingly, when the piston rod comes out of the cylinder with the activation of the working fluid, the swivel bracket pivots about the tilt axis and the outboard drive is tilted up to a lifted position relative to the associated watercraft. When the piston rod comes into the cylinder with the reversed activation of the working fluid, then the outboard drive is tilted down to a lowered position.
Some of the hydraulic tilt devices for relatively small outboard motors have a manually operable switchover valve in a passage that connects the pair of the fluid chambers with each other and an operator of the outboard motor can switch the passage between a communication state and a non-communication state. When the passage is in the communication state, the operator may tilt up or tilt down the outboard drive without any restraint. Meanwhile, by bringing the passage to the non-communication state, the outboard drive will not be moved and stay at any position that the operator desires.
The majority of the hydraulic tilt devices contain gasses in said cylinder or an accumulation chamber, which is provided separately from the cylinder, for compensating the capacity of the working fluid being out of the cylinder, because gasses are compressive.
FIGS. 1 and 2 illustrate cross-sectional side views of some exemplary hydraulic tilt devices which are conventionally used. The tilt devices includes a cylinder 20, a piston 22, a piston rod 24, a passage 26 and a switchover valve 28 which are described above. A working fluid 30 fills both chambers of the cylinder 20. A floating piston 32 is additionally provided in the cylinder 20. This floating piston 32 slides in the cylinder 20 with the piston 24 as a unit except under the condition that a submerged obstacle strikes the outboard motor. In addition, the tilt device shown in FIG. 2 further has an accumulator chamber 34 communicated with a chamber 36 where the piston rod 24 does not extend. The chamber 36, in other words, is the opposite chamber relative to the other chamber 37 where the piston rod 24 extends. Gasses 38 are contained in the chamber 36 or the accumulator chamber 34.
The hydraulic tilt device as such constructed has a problem. When the outboard motor is operated, the thrust force is exerted upon the gasses 38 because the piston rod 24 comes into the cylinder 20 by the thrust force and pushes the unit of the piston 22 and the floating piston 32 toward the gasses 38. Since the gasses are compressive, at some occasion they accept this thrust force and some other occasion they push back. This gives the operator a kind of xe2x80x9cfloating feelingxe2x80x9d and the operator is likely to believe that the operational performance of the outboard motor is not so good.
FIG. 3 illustrates a cross-sectional side view showing another type of tilt device that can be proposed for resolving the problem. In this tilt device, gasses 38 exclusively exert onto the working fluid 30 in the chamber 37 where the piston rod 24 extends when the switchover valve 28 closes the passage 26. Thus, the phenomenon coming with the tilt devices shown in FIGS. 1 and 2 no longer occur with this tilt device. However, another problem arises particularly with this arrangement, not limited to though.
The problem tends to appear when the associated watercraft is running in shallow water. Usually, the drive unit of the outboard motor is lifted up to a tilt range and powered with a low or medium speed in this situation so that submerged obstacles might not strike it. Under the circumstances, if the operator powers up the engine of the outboard motor to run it at a full speed without lowering down the drive unit, the propeller thrust is headed downwardly as well as forwardly. Such a downward thrust, then, raises the bow of the associated watercraft. If this happens abruptly, the watercraft may lose balance.
With the tilt devices shown in FIGS. 1 and 2, the xe2x80x9cfloating feelingxe2x80x9d is amplified when the operator tries to power up the engine and the operator can relatively easily notice that the situation could happen. Accordingly, the problem is not so serious with those tilt devices. However, it is desirable to avoid this problem even in using the tilt devices shown in FIGS. 1 and 2.
Further, the problem may appear even with hydraulic tilt devices that contain no gasses therein but have a reservoir containing surplus fluid for compensating the capacity of the piston rod. This type of hydraulic tilt devices is also conventional.
It is, therefore, a principal object of this invention to provide a hydraulic tilt device thereby a drive unit can be lowered down when an engine is powered up under the condition that the drive unit is tilted up for advancing in a shallow water.
It is another object of this invention to provide a hydraulic tilt device thereby the xe2x80x9cfloating feelingxe2x80x9d does not occur and a drive unit can fall down in the same situation as described above even though the operator do not have the xe2x80x9cfloating feelingxe2x80x9d.
A conventional hydraulic tilt device usually has a shock absorber mechanism thereby an underwater obstacle can be safely cleared. It is desirable to have the relief function in the shallow water running with reserving the function of the shock absorber mechanism.
It is, therefore, a further object of this invention to provide a hydraulic tilt device that can achieve the relief function in the shallow water running as well as the shock absorber function.
This invention is adapted to be embodied in a hydraulic tilt device for a marine outboard drive.
In accordance with one aspect of this invention, the hydraulic tilt device comprises a cylinder. A piston is slidably supported in the cylinder and defining a first chamber and a second chamber in the cylinder. A working fluid is contained in the first and second chambers. A piston rod extends from the piston through the first chamber. A passage is disposed outside of the piston and joins the first and second chambers together. A switchover valve is disposed in the passage for switching the passage between a communication state and a non-communication state. Gasses are contained for compensating the working fluid and pressuring the working fluid in the first chamber when the passage is in the non-communication state. Means are provided for permitting flow of the working fluid from the second chamber to the first chamber when the passage is in the non-communication state and the second chamber produces a pressure greater than a predetermined magnitude.
In accordance with another aspect of this invention, the hydraulic tilt device comprises a cylinder. A piston is slidably supported in the cylinder and defining a first chamber and a second chamber in the cylinder. The first and second chambers are filled with a working fluid. A piston rod extends from the piston through the first chamber. A passage is disposed outside of the piston and joins the first and second chambers together. A switchover valve is disposed in the passage for switching the passage between an open state and a closed state. Means are provided for bypassing the switchover valve when the passage is in the closed state and the second chamber produces a pressure greater than a predetermined magnitude.
In accordance with a further aspect of this invention, a method of operating a hydraulic tilt device is proposable. The hydraulic tilt device includes a cylinder. A piston is slidably supported in the cylinder and defining a first chamber and a second chamber in the cylinder. The first and second chambers are filled with a working fluid. A piston rod extends from the piston through the first chamber. A first passage is disposed outside of the piston and joins the first and second chambers together. A switchover valve is disposed in the first passage for switching the first passage between a communication state and a non-communication state. A second passage bypasses the switchover valve. The method comprises the steps of maintaining the first passage in the non-communication state by the switchover valve, and permitting flow of the working fluid through the second passage when the second chamber produces a pressure greater than a predetermined magnitude.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.